We've heard it said: "Birds of a feather flock together." The phrase of justification is used to gently explain why people with similarities prefer to join a certain group. It's quite unavoidable. Guys like to hang with other guys watching sports or playing golf. Women like to go shopping or to restrooms together. Science fiction fans enjoy huddling together at various conventions. From church denominations, to office teams, to politics, to skin color...people prefer interacting with groups in which they have something in common. Commonality fosters comfort. If there's no comfort, one is less likely to remain in a group.